This invention relates to a correcting lens utilized to form a fluorescent screen of a colour picture tube and a method of manufacturing such a correcting lens and more particularly to a correcting lens wherein the effective surfaces on both sides of the lens are divided into a plurality of regions and a method of manufacturing such a correcting lens.
The fluorescent screen of a shadow mask type colour picture tube is generally formed by regularly coating phosphor dots of three colours by using photographic technique. In this case, for the purpose of forming the phosphor dots an exposure device as diagrammatically shown in FIG. 1 is used. The exposure device 1 shown in FIG. 1 comprises a light source 2, and a correcting lens 3 for causing the light beam emanated from the light source to approximate the actual locus of the electron beam and the coating of the phosphors applied on the inner surface of the panel 5 of a colour picture tube is exposed to the light beam emanated by the light source 2 through the correcting lens 3 and a shadow mask 4 supported in front of the phosphor coating. Such an exposure device has been used not only to form phosphor dots but also to form so-called matrix holes which are used to form the phosphor dots of a black matrix type colour picture tube. The novel correcting lens can be used in the manufacture of these two types of fluorescent screens.
The surface of the prior art correcting lens utilized in such an exposure device includes extremely complicated curved surfaces for the purpose of causing the light beam to substantially approximate the locus of the electron beam. Continuously curved surfaces are generally used, but with this construction it has been impossible to cause the exposure light to approximate the locus of the actual electron beam beyond a certain limit. For this reason, it has been impossible to perfectly align the phosphor dots and the electron beam spots over the entire area of the panel. Accordingly, in the colour picture tube having a fluorescent screen formed by using such a correcting lens with continuous curved surfaces it is inevitable to form a colour shading thus imparing the picture quality.
To obviate these difficulties it has been proposed to provide a correcting lens as shown in FIGS. 2 and 3 wherein the effective area 6a1 of the side 6a of the lens is divided into a plurality of plane or curved regions 601, 602, . . ., 6nn by means of discontinuous border lines 6x1 - 6xn and 6y1 - 6yn. The effective area 6bl of the opposite side 6b is plane.
When compared with correcting lenses having continuous curved surfaces or having stepped discontinuous portions in the radial direction on one side, the correcting lens shown in FIGS. 2 and 3 can more accurately coincides the exposure light beam and the locus of the actual electron beam but due to the discontinuous border lines 6x1 - 6xn and 6y1 - 6y2 between respective regions 601, 602, . . ., 6nn a grid shaped nonuniform exposure pattern is formed. Accordingly, a fluorescent screen 5 formed by using such a correcting lens 6 has a nonuniform appearance as at 5a as shown in FIG. 4 even when the fluorescent screen is not in operation. When the areas of the divided regions of the correcting lens are small the effect of such a nonuniform appearance is not significant, but it is difficult to decrease the areas beyond a predetermined limit for various reasons. Furthermore, when the number of the divided regions increases to 100 or several hundreds, it is necessary to use a number of process steps thus increasing the cost of manufacturing.
Even when a lens having discontinuous surfaces is combined with a lens having continuously curved surfaces it has been impossible to completely solve the problem of nonuniform appearance.